A structure of unspecified size and shape, either stationary or moving, has some type of internal excitation. This internal excitation energy is passed through the structure until it reaches the shell of the structure. The energy on the shell then dissipates into the surrounding field and can be regarded as emanating acoustic waves. The acoustic waves on the surface of the structure are either radiating waves or non-radiating (i.e., evanescent) waves and exist in a region known as the evanescent acoustic near field. The radiating acoustic waves cause observable responses in the acoustic far field. For many underwater structures, it is desirable to reduce the observable acoustic radiation responses in the acoustic far field of a radiating structure.
In attempting to reduce the far field acoustic radiation from a structure using an active control system, a measurement of the amount of acoustic radiation in the acoustic far field is desirable as a means of ascertaining the performance of the active control system. In a typical application, this is not measured and is often times unmeasurable. For instance, if the structure is a moving vehicle, it is difficult and impractical to place acoustic measurement devices in the acoustic far field to measure the acoustic radiation from the vehicle.
Current active control methods circumvent this problem of actually measuring the far field radiation by focusing on vibrations within the structure. The active control system essentially attempts to minimize the vibrations of the structure, and the system's performance criterion is based on measurements of the structure's vibrations. However, by attempting to reduce the vibrations of a structure, the main objective of reducing the far field acoustic radiation levels can be achieved with certainty only if all the vibrations of the structure are reduced.
More specifically, in structural acoustic active control, the active control system reduces the outputs of the structure's modes, which are analyzed from a structural acoustics point of view. Modes are a means of classifying the frequency dependent vibrations of a structure. When a structure is excited, the modes of the structure are said to be excited and some of these modes cause acoustic radiation to the far field while some do not. With structural acoustic active control, energy to control the structure's vibrations is placed directly into the structure with the purpose of reducing the modal contributions to the acoustic radiation in the far field. Thus, intimate knowledge of the structure's modes is required to apply structural acoustic active control. However, as the complexity of the structure increases, this knowledge is more difficult and expensive to obtain.